1. Field of the Invention
This invention relates to plasma processing apparatus and in particular, but not exclusively, to inductively coupled plasma helicon or electron cyclotron resonance apparatus.
2. Description of Related Art
An early suggestion for the design of an inductively coupled plasma apparatus is described in U.S. Pat. No. 4,810,935, in which an antenna operates within an axial magnetic field upon a plasma source, which is coupled to a larger volume process chamber. This noted that, under these conditions, efficient coupling of the RF power could be achieved. This efficiency is due to the resonant response of the electrons' motions in the form of helicon plasma waves. The concept was further developed in U.S. Pat. No. 4,990,229, and other examples of helicon plasma source designs are contained in U.S. Pat. Nos. 5,449,433, 5,567,268 and 6,189,484.
In an alternative approach, power is supplied radiatively to the plasma to generate an electron cyclotron resonance (ECR), as described in U.S. Pat. No. 3,418,206. Here, high power transfer efficiency results from the resonant response of the electrons motions in the form of cyclotron orbits around the magnetic field. Some examples of the use of ECR sources for plasma processing can be found in U.S. Pat. Nos. 4,401,054, 4,609,428 and 4,638,216.
In both helicon and ECR designs, to optimise power absorption efficiency strong overlap, extending over a large volume of plasma, is required between the source electromagnetic fields and the electron motions at the excitation frequency.
Many possible RF antennae geometries can be used to excite plasma waves in a helicon source. These include the single loop and two loop antenna of U.S. Pat. No. 4,990,229, which couple to axially symmetric (m=0) wave modes, as well as geometries that couple to m=+1 modes. Practical antennae are of finite length and are therefore able to produce a range of wave numbers along the antenna axis (which is also the magnetic field direction). The coupling efficiency at each wave number will be determined in part by the antenna geometry, but also by the geometry of the wave modes, which is influenced by the chamber walls, the magnetic field profile and the electron density profile in the vicinity of the antenna. Strong variations in field strength or electron density affect the wave propagation and cause strong spatial variations in the amplitude and wave number, making it difficult to couple efficiently using a simple antenna geometry. There will nevertheless be a characteristic wave mode for which the coupling is strongest, and this will determine the power coupling efficiency of the antenna. In the prior art this is not especially well controlled or understood, because the field and plasma density vary strongly along the field direction.
In an electron cyclotron resonance (ECR) source, the microwave field geometry is determined by the launching wave guide. Power coupling is to the cyclotron motion of the electrons and efficient coupling requires that the cyclotron frequency closely matches the excitation frequency. If the D.C Magnetic field strength is strongly varying, this can only be achieved over a small volume of space. In both the helicon and ECR sources, therefore, there is a need for improved control and uniformity in the magnetic field in the source region.